You may very well be correct about the furnace being used to heat water, but I don't think that is a good solution to your problem. First, it sound as though your current water heater has an element out. It could be too small, but even a small heater should perform better that you describe. First thing I'd do is to get the heater checked out, fix it if necessary, and see if that will take care of the problem. If it still won't supply enough water, then a new heater that is large enough to supply your needs would be the best way to go.